1. Field of the Invention
The invention relates to printing, and in particular to a method of screen printing on hard non-absorbent materials such as glass, and screens for use in such a method.
An important application of screen printing is in the printing of heater arrays comprising electrically conducting networks on rear windows for motor vehicles, so that the surface of the window can be heated to demist or de-ice the window. Such heater arrays on the glass generally comprise an array of heating conductors extending across the window between bus bars adjacent the lateral edges of the glass.
The heater array may be printed on one surface of a glass pane which is to form the vehicle window, before the pane is heated for bending and toughening, using an ink which is a mixture of glass frit, silver and binder material. The heater pattern is screen printed using a screen in which the required pattern is defined by a coating on the screen surface, and is fired on to the glass surface when the glass sheet is heated for bending and toughening; the binder is generally burnt off during the firing process. The ink may include a resin which is curable by ultra-violet light so that the printed heater array can be cured by brief exposure to ultra-violet light before handling.
2. Description of the Prior Art
It is desirable to be able to increase the thickness of the printed ink pattern at one or more locations, for example to selectively increase the conductance of parts of the pattern such as the bus bars, or to provide a thicker conductive layer where an electric connector is to be soldered on to a bus bar. It is possible to increase the thickness of the pattern in selected locations by printing a second layer of ink over those pattern areas in which the increased thickness is required using an appropriately patterned screen. However, it is difficult in practice to achieve the necessary registration between the two prints, and, because of the need for a second pass through a printing process, additional production costs are incurred.
British Pat. specification No. GB 1 262 176 relates to the screen printing of electronic circuits using metal screens in place of woven fabric screens. The required pattern is printed through openings in the screen, which thus acts as a stencil for the pattern. In order to print pattern areas of varying thickness, it is proposed to incorporate a metal mesh in the openings where an ink layer of reduced thickness is required. This mesh occupies a part of the volume which would otherwise be available for the paste or ink being printed on the substrate, and leads to a reduction in the thickness of the pattern printed in such areas.
European Pat. specification No. EP 0 193 464 A2 refers to two methods for selectively increasing the thickness of certain areas of a conductive pattern to be used as a heater array on a vehicle window. In the first method, said to be described in DE.TM.OS 32 31 382, a screen is used which has thicker threads, for example thicker weft threads, in those areas in which a thicker conductive layer is required. In the second method, the subject of EP 0 193 464 A2, a screen is used which has a coarser mesh in those areas in which a thicker conductive layer is required; this coarser mesh is achieved by omitting alternate weft threads from the screen material in those areas.
The two methods referred to in the European patent specification both suffer from the disadvantage that all parts of the patterns in the areas in which the weft (or warp) fibres are modified or omitted are increased in thickness. It is desirable to have a more versatile method in which any desired areas, or all areas of the printed pattern can be increased in thickness.
In British Pat. specification No. GB 1 307 535, this problem is avoided by using a silk screen on which a patterned emulsion coating defining the pattern to be printed is of increased thickness in areas surrounding and defining the areas in which increased ink thickness is required. However, in practice this method has not proved very successful, especially when printing wide bands of ink, such as the bus bars of heater arrays on vehicle backlights.